遇到一個(gè)筆試題：5個(gè)線程內(nèi)部打印hello和word，hello在前，要求提供一種方法使得5個(gè)線程先全部打印出hello后再打印5個(gè)word

首先想到的是CountDownLatch、CyclicBarrier這類(lèi)線程阻塞工具，趁此機(jī)會(huì)，總結(jié)下這幾個(gè)工具的用法吧。

1、CountDownLatch

先看下源碼吧，這是構(gòu)造方法

/**
     * Constructs a {@code CountDownLatch} initialized with the given count.
     *
     * @param count the number of times {@link #countDown} must be invoked
     *        before threads can pass through {@link #await}
     * @throws IllegalArgumentException if {@code count} is negative
     */
    public CountDownLatch(int count) {//count為計(jì)數(shù)值  
        if (count < 0) throw new IllegalArgumentException("count < 0");
        this.sync = new Sync(count);
    }

還有幾個(gè)主要的方法

public void await() throws InterruptedException {};   //調(diào)用await()方法后線程會(huì)被掛起，等待直到count值為0才繼續(xù)執(zhí)行，除非線程Intercept
public boolean await(long timeout, TimeUnit unit) throws InterruptedException {};  //和await()類(lèi)似，只不過(guò)等待一定的時(shí)間后count值還沒(méi)變?yōu)?的話(huà)就會(huì)繼續(xù)執(zhí)行  
public void countDown() {};  //將count值減1  

先寫(xiě)一個(gè)簡(jiǎn)單的例子，比如5個(gè)線程都打印完HelloWorld 后，打印一條語(yǔ)句

    public static void main(String[] args) {
        CountDownLatch latch = new CountDownLatch(5);
        for (int i = 0; i < 5; i++) {
            new PrintThread(latch, i).start();
        }
        try {
            latch.await();//等待，直到此CountDownLatch里面的count=0
        } catch (InterruptedException e) {
            e.printStackTrace();
        }
        System.out.println("******** Print Over ***********");

    }

    static class PrintThread extends Thread {
        CountDownLatch latch;
        int id;

        public PrintThread(CountDownLatch latch, int id) {
            this.latch = latch;
            this.id = id;
        }

        @Override
        public void run() {
            try {
                sleep(1000);
            } catch (InterruptedException e) {
                e.printStackTrace();
            }
            System.out.println("Hello World " + id);
            this.latch.countDown();//在做完工作后，countDown減1
        }
    }

結(jié)果如下

Hello World 0
Hello World 3
Hello World 2
Hello World 1
Hello World 4
******** Print Over ***********

工作線程每次執(zhí)行完后，countDown()減1，await()掛起，先在那等著，直到count=0的時(shí)候，才繼續(xù)向下執(zhí)行。
好了，回到開(kāi)始的題目，稍微改下就可以了

    public static void main(String[] args) {
        CountDownLatch latch = new CountDownLatch(5);
        for (int i = 0; i < 5; i++) {
            new PrintThread(latch, i).start();
        }
    }

    static class PrintThread extends Thread {
        CountDownLatch latch;
        int id;

        public PrintThread(CountDownLatch latch, int id) {
            this.latch = latch;
            this.id = id;
        }

        @Override
        public void run() {
            System.out.println("Hello " + id);
            this.latch.countDown();
            try {
                this.latch.await();
            } catch (InterruptedException e) {
                e.printStackTrace();
            }
            System.out.println("World " + id);
        }
    }

結(jié)果如下

Hello 0
Hello 3
Hello 4
Hello 2
Hello 1
World 1
World 2
World 3
World 4
World 0

同一個(gè)CountDownLatch對(duì)象，在線程中，打印完第一條"Hello"語(yǔ)句后，countDown()，然后await()掛起，count=0的時(shí)候，再向下打印"World" 語(yǔ)句

2、CyclicBarrier

先看下構(gòu)造方法

  /**
     * Creates a new <tt>CyclicBarrier</tt> that will trip when the
     * given number of parties (threads) are waiting upon it, and
     * does not perform a predefined action when the barrier is tripped.
     *
     * @param parties the number of threads that must invoke {@link #await}
     *        before the barrier is tripped
     * @throws IllegalArgumentException if {@code parties} is less than 1
     */
    public CyclicBarrier(int parties) {
        this(parties, null);
    }
   /**
     * Creates a new <tt>CyclicBarrier</tt> that will trip when the
     * given number of parties (threads) are waiting upon it, and which
     * will execute the given barrier action when the barrier is tripped,
     * performed by the last thread entering the barrier.
     *
     * @param parties the number of threads that must invoke {@link #await}
     *        before the barrier is tripped
     * @param barrierAction the command to execute when the barrier is
     *        tripped, or {@code null} if there is no action
     * @throws IllegalArgumentException if {@code parties} is less than 1
     */
    public CyclicBarrier(int parties, Runnable barrierAction) {
        if (parties <= 0) throw new IllegalArgumentException();
        this.parties = parties;
        this.count = parties;
        this.barrierCommand = barrierAction;
    }

主要方法是

public int await() throws InterruptedException, BrokenBarrierException {};//掛起當(dāng)前線程，直至所有線程都到達(dá)阻塞狀態(tài)（即所有線程都執(zhí)行到了await方法）再執(zhí)行await后面的任務(wù)；  
public int await(long timeout, TimeUnit unit)throws InterruptedException,BrokenBarrierException,TimeoutException {};//讓這些線程等待至一定的時(shí)間，如果還有線程沒(méi)有到達(dá)阻塞狀態(tài)就直接讓已經(jīng)到達(dá)阻塞狀態(tài)的線程執(zhí)行后續(xù)任務(wù)  

回到最開(kāi)始的題目

public static void main(String[] args) {
        CyclicBarrier cyclicBarrier = new CyclicBarrier(5);
        for (int i = 0; i < 5; i++) {
            new PrintThread(cyclicBarrier, i ).start();
        }
        System.out.println("******** Print Main ***********");
    }

    static class PrintThread extends Thread {
        CyclicBarrier cyclicBarrier;
        int id;

        public PrintThread(CyclicBarrier cyclicBarrier, int id) {
            this.cyclicBarrier = cyclicBarrier;
            this.id = id;
        }

        @Override
        public void run() {
            System.out.println("Hello " + id);
            try {
                this.cyclicBarrier.await();
            } catch (InterruptedException | BrokenBarrierException e) {
                e.printStackTrace();
            }
            System.out.println("World " + id);
        }
    }

執(zhí)行結(jié)果如下

Hello 0
******** Print Main ***********
Hello 3
Hello 2
Hello 1
Hello 4
World 0
World 2
World 3
World 4
World 1

在執(zhí)行完打印"Hello"任務(wù)后，調(diào)用CyclicBarrier的await()方法，線程阻塞在這里，等所有線程都await()了，再執(zhí)行以下的任務(wù)，打印"World"。
另外，CyclicBarrier第二個(gè)構(gòu)造方法里有一個(gè)Runnable類(lèi)型的參數(shù)，看下注釋?zhuān)?/p>

// 等阻塞釋放了，再執(zhí)行的指令
@param barrierAction the command to execute when the barrier is tripped 

我們用另外一個(gè)構(gòu)造方法試試

CyclicBarrier cyclicBarrier = new CyclicBarrier(5, new Runnable() {

            @Override
            public void run() {
                System.out.println("******** Print Main ***********");
            }
        });

執(zhí)行結(jié)果如下

Hello 0
Hello 2
Hello 1
Hello 4
Hello 3
******** Print Main ***********
World 3
World 0
World 1
World 2
World 4

所有的"Hello" 打印完，再執(zhí)行這個(gè)barrierAction參數(shù)，而且，需要注意的是，等barrierAction執(zhí)行完畢后，才會(huì)再往下執(zhí)行。

總結(jié)

CountDownLatch、CyclicBarrier都能實(shí)現(xiàn)線程阻塞，區(qū)別是在使用過(guò)程中CountDownLatch調(diào)用await()方法僅實(shí)現(xiàn)阻塞，對(duì)計(jì)數(shù)沒(méi)有影響，線程執(zhí)行完需要手動(dòng)countDown()更新計(jì)數(shù)，而CyclicBarrier調(diào)用await()方法后，會(huì)統(tǒng)計(jì)是否全部await()了，如果沒(méi)有全部await()，就阻塞，也就是說(shuō)，CyclicBarrier的await()方法也參與計(jì)數(shù)了；另外一方面，CountDownLatch不能復(fù)用，CyclicBarrier可以在某個(gè)線程中調(diào)用它的reset()方法復(fù)用。

補(bǔ)充

CountDownLatch、CyclicBarrier和后面要講的Semaphore、JDK 1.6版本的FutureTask都是基于AbstractQueuedSynchronizer機(jī)制實(shí)現(xiàn)同步的，源碼解析可參考：http://brokendreams.iteye.com/blog/2250372